There is great interest in a new generation of electronic devices which use organic semiconductors as their active components, such as organic light-emitting transistors (OLETs) and organic photovoltaics (OPVs). In particular, there is interest in conjugated polymer based organic semiconductors because they combine the electrical properties of semiconductors with the mechanical properties of plastics. Furthermore, since these materials can be processed relatively inexpensively with techniques such as spin-coating and ink jet printing, conjugated polymers are finding increased applications in optoelectronic devices such as plastic light-emitting diodes (LEDs) and photovoltaic cells. With an ability to form active layers in these types of electronic devices, conjugated polymers provide promising materials for optimizing the performance of existing devices as well as the development of new devices.
Charge mobility is one of the most important factors in the performance of semiconducting polymers for use in thin-film transistors (TFTs) [1, 2] and photovoltaic cells [3-5]. The effort to boost the charge-carrier mobility of conjugated polymers has spanned over thirty years [33, 42-44]. Ideal semiconducting polymers must exhibit high mobility to be competitive in TFTs and photovoltaic cells, where rapid charge transport is crucial to device performance. In polymer light-emitting diodes [6-8], electrochemical cells [9], and biosensors [10], however, efficient light emission is the critical factor, but charge transport is also essential. Due to the high degree of conformational freedom of macromolecular chains and the irregular interchain entanglement, polymers tend to form disordered structures at nanometer to micrometer length scales. This disorder impedes charge transport. High charge mobility can only be obtained when the polymer chains align in a linear configuration favorable for charge delocalization along the conjugated backbone. In order to allow efficient long-range transport within a polymer film, microscopic molecular packing and macroscopic anisotropic alignment must be created.
A great deal of effort has been devoted to designing new materials in order to induce anisotropic alignment of polymer chains by strengthening intermolecular interactions, such as hydrogen bonding [11], sulphur-fluorine interactions [2], and π-π stacking [12, 13]. So far, however, semiconducting polymers have not been demonstrated to self-assemble into large-scale, ordered structures. There have been many attempts to improve charge transport mobility through material processing, such as the application of shear force with the use of doctor blading [14], dip coating [15], strain stretching [16], Langmuir-Blodgett deposition [17], and topographical patterning [18]. Although these processing methods have demonstrated progress toward molecular assembly and chain alignment, the measured TFT mobilities have remained insufficient for most applications (typically less than 3 cm2V−1 s−1) [2, 15, 18].
There is no solution-based processing technology that is capable of creating semiconducting polymer thin-films showing self-assembly of aligned nanocrystalline domains with charge transport mobilities comparable to values obtained in inorganic semiconductors. A strategy to macroscopically self-assemble semiconducting polymers and thereby harness their unique potential for anisotropic charge transport is needed for enhancing their performance and accelerating their applications in technologies such as optoelectronics.